Field of the Invention
The present invention relates to a method of probing test for aligning a large number of probes relative to the positions of a large number of pads of IC chips on a semiconductor wafer.
Any conventional probing test machine (hereinafter merely called a "prober") available for a power-supply test performed against circuitry of an IC chip incorporates a loading/unloading section, a probing section, and a testing section, respectively. The loading/unloading section incorporates an alignment stage. The probing section incorporates a wafer stage and a probe card. The testing section incorporates a wafer stage and a probe card. The testing section incorporates a tester for applying test signals.
In order to automatically execute tests by operating a wafer prober, first, the wafer prober extracts a semiconductor wafer from a cassette stored in the loading/unloading section, and then delivers the extracted semiconductor wafer to the alignment stage before executing a preliminary alignment (which is substantially a process to properly position an orientation flat) of the semiconductor wafer on the alignment stage. After completing the preliminary alignment, the semiconductor wafer is transferred to the wafer stage of the probing section, and then the wafer prober executes a process to properly position the array of rectangular pads (eg. length of side of pad is 60 to 100 .mu.m) formed on an IC chip on the semiconductor wafer relative to the array of probes. After completing this positioning process, all the probes are brought into contact with those corresponding pads to allow specific current to flow through them, and then the wafer prober transmits specific test signals to the tester of the testing section. In response to the received test signals, the tester automatically identifies whether the tested IC chips are acceptable, or not.
When executing a probe test, a dummy wafer is used, where the position of a wafer card is preliminarily corrected so that the position of the wafer card can be held in each of X, Y, Z, and .theta. directions, against the wafer stage. The term "dummy wafer" designates such a wafer available for executing a simulation test, which is made of aluminium-evaporated pattern exactly identical to the actual pattern of the semiconductor wafer which is designated to be a commercial product.
A memory of the prober preliminarily stores data on the arrangement of all the pads of the IC chip. Based on this data on the arrangement of all the pads, the prober automatically executes probing tests against IC chips available for industrial use.
The semiconductor wafer is preliminarily aligned on the alignment stage. Then, the preliminarily aligned wafer is conveyed onto the wafer stage. Next the wafer stage is moved to the probe position represented by the data stored in memory. There is still a slight positional deviation between the probes and the pads. In order to bring the probes into contact with the pads, the operator must measure the deviation and eliminate the deviation between the probes and the pads.
To eliminate the deviation the operator normally adjusts, the position of the wafer stage until the probes come into contact with the corresponding pads. The data on the adjusted position of the wafer stage is stored into the memory of the prober. This process is conventionally called "teaching".
Whenever changing the available specification and the kind of the semiconductor wafer, an applicable probe card is also replaced, thus obliging the operator to execute a "teaching" operation whenever changing the probe card. After completing the "teaching" operation, the prober transfers the wafer stage by a distance corresponding to a pitch between IC chips, and then automatically and sequentially measures the positions of the IC chips. After completing a test on a semiconductor wafer, this wafer is brought back to the cassette, and then, the wafer prober conveys the next semiconductor wafer onto the wafer stage for the next test.
When measuring the deviant positions between the pads and the probes, a number of probes are brought into contact with the dummy wafer so that a number of fine recesses can be generated on the surface of the dummy wafer, and then, using these fine recesses, the wafer prober computes the deviation of the positions between those probes and pads. Concretely, based on the data on the positions of these recesses, the wafer prober identifies the amount of deviation present between the actual positions of the probes and the pads and those positions of the second, third, fourth probes on, which ought to be at the normal positions, respectively.
Based on the computed amount of the deviant positions between those probes and pads, the operator rotates the probe card by a certain amount corresponding to minus .theta. in order to correct the position of the probe card. Next, the operator mounts the wafer (which is subject to the initial measurement and a preliminary alignment process on the alignment stage) on the wafer stage. Note that, after turning the semiconductor wafer by an angle .theta., the position of the semiconductor wafer relative to the probe is already corrected as of this time. Next, the operator visually corrects the position of the wafer stage with a microscope. More particularly, the prober operator shifts the wafer stage in the directions of axes X and Y up to a specific position where a reference pad can be brought into contact with a reference probe of the IC chip by manually operating the wafer-stage driving mechanism. In the final step, data on the position of the wafer stage is stored in memory of the wafer prober to complete a "teaching" operation. Next, based on the value yielded from the "teaching" operation, the wafer prober convey the wafer before automatically executing a probe test.
On the other hand, since about 500 pads are formed on a single IC chip, it is extremely difficult for the prober operator to correctly identify the position of the reference probe relative to the position of the reference pad by directly and visually checking all the pads on the IC chip with a microscope. In other words, it is extremely difficult for the operator to correctly identify the reference pad from an extremely large number of pads and then bring the reference probe into contact with the reference pad, and yet, it takes a very long while before correctly identifying the reference pad. As a result, the operator often incorrectly identifies the reference pad.